Osteoarthritis is a disease of aging and is directly related to the stresses of weight bearing, mechanical overuse or injury to the joint. Both joint space cartilage and the subchondral bone are implicated in the disease process. The role of the bone in initiating and/or propagating this process is not clearly understood. More than thirty years ago Radin and other researchers proposed that the changes in bone might be the predisposing event leading to the development of osteoarthritis. Today, although it is recognized that bony changes are a prominent feature of OA, we remain uncertain of its causal relationship to this disease. In osteoarthritis, it is believed that the bone immediately below the articular cartilage (the subchondral bone) is prone to stress injury, which leads to microcracks in the bone. These micro crack may lead to the death of bone cells, and the initiation of the bone remodeling process. This bone remodeling results in the formation of bony over-growth in the form of mineralized callus at the bone injury site. These calluses increase the bone volume and stiffness, when compared to normal OSTEOPONTIN: Osteopontin is a specialized protein like substance (phosphorylated and sulphated glycoprotein). It can be found in a variety of tissues; e.g., bone forming cells, primitive cells from the uterus, placenta, kidney, and nervous system. It has also been found in activated macrophage and lymphocytes. This protein plays an important role in bone formation. In bone, osteopontin is felt to send signals to the cells in response to mechanical stimuli. In the embryonic tissue, OPN is detected in the bone-forming and cartilage cells. It is usually absent in normal healthy adult cartilage. However, in osteoarthritis, OPN could be detected in the cartilage and it?s expression increase with the severity of the disease. It can also be found in the bone forming cells (osteoblasts) of the subchondral bone. Osteopontin production by the bone remodeling cells plays an important role in new bone formation. Osteopontin role may be to facilitate cell attachment to the mineral component of the bone. It also interacts with other bone forming components like type I collagen, osteocalcin, and fibronectin. Mechanical forces stimulate OPN production by acting on the cell to produce shear stress at its adhesion site. This stress transmits a signal to the cell, resulting in OPN production. The final result is a change in the shape of the cell and how it responds to (or interacts with) its environment. The objective of this proposal is to use an osteopontin (OPN) knockout mouse model to examine the role of cartilage and bone forming cells in modulating the development of exercise-induced OA. It is our premise that the OPN-deficient (OPN -/-) mouse may be an ideal model to study chondrocyte viability in articular cartilage, and the role of the subchondral bone alterations in knee osteoarthritis. This study protocol was submitted for review three weeks ago, and was approved with minor revisions on 8/28/04. The budget was approved for the purchase of the mice exercise system, and the order placed five days ago. The experiments will begin within the next four weeks. Goals during the next year: This study will use a total one hundred and fifty six animals; divided into four groups(Table #1). There will be seventy-eight OPN -/- and 78 wild type mice. In each of the four groups (2 exercise, and 2 non-exercise; with and without, OPN) a total of 36 animals will be tested (six animals at each of the six time points). Animals will be separated into groups immediately after weaning (~ 4 weeks) and the exercise will begin when animals are eight weeks of age and continue until the animals are 68weeks old. Knee joint CT, MRI, chondrocyte and subchondral bone cell isolation, and histological studies will be performed on 8 wk old (6 OPN-/- and 6 WT) mice to establish a baseline prior to wheel exercise. The remaining animals will be evaluated as shown in table #1. Six mice per group are required for statistical significance in results based on similar types of studies. Animal age Exercise: OPN-/-exercise (#) WT exercise (#)*OPN -/- *WT 8 wk 0 wk 6 6 16 8 6 6 24 16 32 24 44 36 56 48 Note: Table 1: is an overview of the proposed animal use. Thirty-six animals will be entered and studied in each of the four groups. Animals are selected for the study groups at the time of weaning (~ 4 weeks of age). At the determined time points (0, 8, 16, 24, 36, 48, and 60 weeks), six animals from each group will be euthanised and evaluated as described.